In many industrial chemical processes, the amount of reactants, or input components, that are used is less than or more than the amount necessary to carry the reaction to the point of obtaining a desired characteristic(s) of the product stream. If too little of the input component is used, often the desired target value of a characteristic from the process is not obtained. Alternatively, if an excessive amount of an input component is used, the desired characteristic may be obtained, but the excess input component is typically released as waste in the effluent of the process. In other cases, excessive amounts of an input component may cause undesirable reactions to occur that produce unwanted characteristics. Further, the wasted input component is economically costly and can become an environmental pollutant if it is released into the environment without being removed or recycled from the effluent.
The difficulty in controlling chemical processes, such as bleaching, in the pulp and paper manufacturing industry can be caused by a number of factors including qualitative and quantitative variability of the pulp or wood furnish, the composition of the process chemicals, and the consistency (% wood or pulp) of the furnish. Further, changing market requirements for paper products may require a paper manufacturing operation to produce a wide variety of paper grades. New paper processing methods, equipment, and chemicals force the paper bleaching operation to adapt to these technical changes while still monitoring various characteristics of the pulp.
It is therefore desirable to be able to precisely control the input components to obtain the desired target characteristic(s) with little waste. To obtain this control, a characteristic of the effluent of an industrial process should be precisely monitored in real time in order to provide feedback control on the amount of input components which should be added to the reactor to avoid under use or excessive use, and waste, of the input component.
For example, in the pulp and paper industry, hydrogen peroxide and the hydroperoxy anion (HO.sub.2.sup.-) are important input components for the oxidation and bleaching of wood pulps. In a typical pulp bleaching plant situation, the control of the bleaching chemicals is based on the brightness of the incoming pulp, the pulp flow, and the target brightness that is to be achieved. The factors of incoming pulp brightness, pulp flow, and target brightness are then used to calculate the amount of bleaching chemicals required to be added to the pulp to achieve a certain final target brightness. In another system, the brightness of the pulp is measured after bleaching chemicals are added and after allowing the reaction to occur for a defined reaction time. The resultant brightness value of the reaction is then measured and is used for feedback regulation of the bleaching chemicals.
Typically with these feedback systems, the amount of hydrogen peroxide that is used exceeds or overshoots the amount necessary to reach a final target characteristic, such as pulp final target brightness, yellowness, residual peroxide, brightness efficiency, yellowness efficiency, and delignification efficiency. The resultant unwanted variation in these pulp characteristics may cause additional processing problems in the pulp and paper processing mill. Further, in the case of peroxide bleaching, excessive use of hydrogen peroxide results in waste hydrogen peroxide in the pulp effluent, which is both costly and environmentally harmful.
In order to solve these problems the prior art has offered various solutions. For example, U.S. Pat. No. 4,878,998 teaches a method for bleaching of mechanical, thermomechanical and chemi-mechanical pulps whereby peroxide bleaching is controlled by the addition of a preset amount of bleaching chemicals at a first bleaching stage, measuring the brightness of the pulp, feed forwardly adjusting the amount of bleaching chemicals to be added at a second bleaching stage as a function of the measured brightness of the pulp from the first stage, and then bleaching the pulp at the second stage.
Canadian Patent No. 2,081,907 teaches a method and apparatus for determining information characteristics of the concentration of each of at least three intermixed components in kraft liquors having the steps of identifying detectable characteristics that are detectable in relation to the concentration of the components, developing a mathematical relationship between the component and the characteristics, such as regression analysis, analysing a sample of solution with a LTV detector, and then controlling the concentration of each of the three components by using the information from the analysis of the sample.
While current brightness sensors are able to provide a measure of the pulp brightness, they are unable to measure the bleaching efficiency of the bleaching reaction itself. (Bleaching efficiency is the change in brightness of the pulp divided by the residual peroxide in the pulp effluent.) Further, measurement of yellowness efficiency (the change in pulp yellowness divided by the residual peroxide in the pulp effluent) also requires a method by which the residual peroxide in the pulp effluent can be measured.
Unfortunately, it is well known that there is a present lack of an appropriate method or device for the monitoring and control of pulp bleaching reaction characteristics, including pulp final target brightness, yellowness, residual peroxide, brightness efficiency, yellowness efficiency, and delignification efficiency. Also, it is known that pH measurement probes and electrochemical methods of measuring hydrogen peroxide, such as the Kajaani Polarox sensor made by Valmet Automation, can be unreliable under pH conditions which are typically used for pulp brightening reactions.